Techniques for detecting and correcting errors on a ring oscillator

ABSTRACT

A circuit may include a ring oscillator circuit and monitoring circuitry. The ring oscillator circuit has a group of inverters in a loop, whereby the group of inverters includes first, second, and third output nodes. The monitoring circuitry may monitor for error events in a signal that has passed through the inverters from any one of the first, second, or third output nodes, and may generate first and second monitoring circuitry outputs. The circuit may further include an error correction circuit that produces an error correction output based on the first and second monitoring circuitry outputs. Accordingly, the monitoring circuitry may generate first and second updated monitoring circuitry outputs based on the error correction output. The first and second updated monitoring circuitry outputs may be logically combined using a logic circuit to reset the signal that has passed through the loop.

FIELD OF THE DISCLOSURE

The present disclosure relates to electrical circuits, and moreparticularly, to techniques for detecting and correcting errors on aring oscillator.

BACKGROUND

Ring oscillators are used in a variety of circuit applications. Forexample, ring oscillators may be used in clock circuits anddelay-locked-loop-type circuits. A conventional ring oscillator isformed from a loop of inverters. There may be, for example, hundreds ofinverters in the ring oscillator, each of which has its output connectedto the input of another one of the inverters. In some designs, a NANDgate may be inserted into the loop in place of one of the inverters. Oneof the NAND gate inputs may be used as an enable input. The ringoscillator may be enabled by asserting a trigger signal on the enableinput. When the value of the trigger signal on the enable input is low,the ring oscillator will be turned off and will not oscillate. When thevalue of the trigger signal on the enable input is high, the ringoscillator will be enabled and will oscillate.

The trigger signals that are used for enabling and disabling ringoscillators in this way are generally produced using off-chip testequipment. As the trigger signal is routed to the enable input of theNAND gate through interconnects, the trigger signal can become degraded.In particular, a square wave trigger signal may pick up undesirableringing characteristics due to parasitic circuit elements or due topower supply glitches. The spikes or other noise characteristics thatare present in a trigger signal that has been degraded in this way maycause a ring oscillator to enter undesirable modes of operation in whichhigher-order harmonics propagate around the loop. When this occurs, theoperation of the ring oscillator may be unstable or the output of thering oscillator may oscillate at an undesired higher-order harmonicfrequency rather than at an intended fundamental frequency.

SUMMARY

According to some embodiments of the present invention, techniques fordetecting and correcting errors in a ring oscillator are provided.Embodiments of the present invention can be implemented in numerousways, such as a process, an apparatus, a system, or a device. Severalembodiments of the present invention are described below.

According to some embodiments, a circuit includes a ring oscillatorcircuit and monitoring circuitry. The ring oscillator circuit has agroup of inverters in a loop, whereby the group of inverters includesfirst, second, and third output nodes. The monitoring circuitry monitorsfor an error event in a signal that has passed through the invertersfrom any one of the first, second, or third output nodes, and generatesfirst and second monitoring circuitry outputs. The circuit furtherincludes an error correction circuit that produces an error correctionoutput based on the first and second monitoring circuitry outputs.Accordingly, the monitoring circuitry generates first and second updatedmonitoring circuitry outputs based on the error correction output. Thefirst and second updated monitoring circuitry outputs are logicallycombined using a logic circuit to reset the signal that has passedthrough the loop.

According to other embodiments, circuitry includes a ring oscillatorcircuit having first, second, and third output nodes. Each of the first,second, and third output nodes taps a signal that has passed through thering oscillator circuit. The circuitry further includes error detectioncircuitry that detects an error event in the signal through the first,second, and third output nodes, and an error correction circuit thatselectively resets the ring oscillator circuit in response to thedetected error event in the signal. The error detection circuitrygenerates first and second error detection circuit outputs indicatingwhether the error event has occurred between the first, second, andthird output nodes.

According to further embodiments, a method for detecting and correctingerror events in an integrated circuit is disclosed. The method includesmonitoring error effects associated with a signal that propagatesthrough a ring oscillator with error detection circuitry. The errordetection circuitry detects at least one error condition from first,second, or third output nodes of the ring oscillator. The method furtherincludes correcting the error effects associated with the signal whenthe error condition is detected in either the first output node, thesecond output node, or the third output node of the ring oscillator withan error correction circuit coupled to the error detection circuitry.

Further features of the present invention, its nature, and variousadvantages, will be more apparent from the accompanying drawings and thefollowing detailed description of the preferred embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a block diagram of an illustrative ring oscillator circuithaving error detection and error correction circuitry in accordance withan embodiment of the present invention.

FIG. 2 is a circuit diagram of the illustrative ring oscillator circuitof FIG. 1 in accordance with one embodiment of the present invention.

FIG. 3 is a timing diagram illustrating the behavior of signals duringoperation of error detection and correction circuitry on a ringoscillator circuit in accordance with one embodiment of the presentinvention.

FIG. 4 is a flow chart of illustrative steps for detecting andcorrecting errors on an integrated circuit in accordance with oneembodiment of the present invention.

DETAILED DESCRIPTION

The embodiments provided herein include techniques for detecting andcorrecting errors in a ring oscillator. Ring oscillators may be providedon integrated circuits such as digital signal proceSSors, programmablelogic device integrated circuits, microprocessors, application specificintegrated circuits, or other integrated circuits.

In some situations, ring oscillators are incorporated into a circuitdesign. For example, a ring oscillator may be used as part of a clocksignal generator. As another example, a ring oscillator may be used in adelay-locked-loop-type circuit. It should be noted that these are merelyillustrative examples. Ring oscillators may be used in any suitableintegrated circuit application.

However, ring oscillators are prone to generating harmonic frequencies.Harmonic frequencies can be triggered at power-on, by voltage supplynoise, and by single event transients (SETs), which may result in noisespikes or other undesirable signal characteristics in the ringoscillators. If care is not taken, a noisy oscillator signal can cause aring oscillator to oscillate at an undesirable higher-order harmonicfrequency, rather than its intended frequency. This effect can cause acircuit that is based on the ring oscillator to operate improperly.

To avoid these problems, a ring oscillator with error detection andcorrection circuitry may be provided. An illustrative integrated circuitin which a ring oscillator circuit has circuitry for detecting andcorrecting errors is shown in FIG. 1. As illustrated, integrated circuit100 includes ring oscillator circuit 101, monitoring circuitry 103, anderror correction circuitry 105. Ring oscillator circuit 101 may coupleto monitoring circuitry 103 and error correction circuitry 105.Monitoring circuitry 103 is connected to power supplies V_(DD) andV_(SS). For example, the power supplies V_(DD) and V_(SS) may beprovided from a power supply generation unit (not shown) on integratedcircuit 100. Power supply V_(DD) may be a power supply having a higherpotential than power supply V_(SS), and monitoring circuitry 103 andintegrated circuit 100 may receive power from the power supplies V_(DD)and V_(SS) to operate.

In one embodiment, monitoring circuitry 103 may monitor error events inan input trigger signal (e.g., IN signal) that has passed through ringoscillator circuit 101. In response to the error events, errorcorrection circuitry 105 may selectively reset the IN signal that haspassed through ring oscillator circuit 101. The error events may be, forexample, harmonic error events. In one embodiment, monitoring circuitry103 and error correction circuitry 105 may collectively be referred toas error detection and correction circuitry. A more detailed descriptionof monitoring circuitry 103 and error correction circuitry 105 will bedescribed below with reference to FIG. 2.

FIG. 2 is a circuit diagram of illustrative integrated circuit 100 ofFIG. 1 in accordance with one embodiment of the present invention. Asdescribed above, integrated circuit 100 includes ring oscillator circuit101. Ring oscillator circuit 101 may include simple inverting logiccircuits as stages. For example, a group of inverters (highlighted inregion 201) are connected in a loop (or a ring) so that the output ofeach inverter feeds into the input of a successive inverter in the loop.The inverters may be, for example, CMOS inverter circuits or other typesof inverter circuits. A 3-input NAND gate circuit 220 may be interposedin the loop. In this example, ring oscillator circuit 101 may beconfigured to operate as a 31-stage ring oscillator circuit. However, itshould also be appreciated that ring oscillator circuits having adifferent number of circuit stages may be used for the presentinvention.

When integrated circuit 100 is powered and operational, an input triggersignal (e.g., IN signal) that is injected into ring oscillator circuit101 may propagate (or oscillate) through the ring at a predeterminedfrequency. In order to detect error events that occur in the oscillatingsignal, ring oscillator circuit 101 may be tapped at different stages(e.g., the output nodes of inverters 202A, 202B, and 202C) usingmonitoring circuitry 103. Monitoring circuitry 103 may be implemented toserve as error detection circuitry for monitoring and detecting errorevents in ring oscillator circuit 101 based on the tapped output nodesignals (e.g., output node signals N1, N11, and N21) from the outputnodes of inverters 202A, 202B, and 202C. It is noted that monitoringcircuitry 103 and error detection circuitry 103 are used interchangeablyin the description and have the same meaning

In the embodiment shown in FIG. 2, error detection circuitry 103includes two parallel error detection circuits 203A and 203B. Errordetection circuit 203A may detect an error signal if the error eventoccurs between an output node of inverter 202A and an output node ofinverter 202C. Error detection circuit 203B may detect another errorsignal that indicates that the error event has occurred between anoutput node of inverter 202B and an output node of inverter 202C. Stillin FIG. 2, each of error detection circuits 203A and 203B may includetwo register circuits. For example, error detection circuit 203Aincludes register circuits 204A and 206A, and error detection circuit203B includes register circuits 204B and 206B. In a preferredembodiment, register circuits 204A, 204B, 206A, and 206B are D-typeregister circuits.

As an example, in error detection circuit 203A, the D input of registercircuit 204A is tied to power supply V_(DD), and a clock input ofregister circuit 204A is coupled to an output node of inverter 202A. TheQB-output of register circuit 204A and the output node of inverter 202Aare gated to NOR gate 205A. A CLR input is used to clear registercircuit 204A. In a preferred embodiment, CLR is active high. The CLRinput of register circuit 204A is controlled by output node signal N21from the output node of inverter 202C. The D input of register circuit206A is tied to power supply V_(SS), and a clock input of registercircuit 206A is coupled to the output 207A of NOR gate 205A. TheQ-output of register circuit 206A is coupled to an input of errorcorrection circuitry 105. An inverting PRE input is used to presetregister circuit 206A. The inverting PRE input of register circuit 206Ais controlled using an output (e.g., error correction output 208) oferror correction circuitry 105. In error detection circuit 203B, theclock input of register circuit 204B is coupled to an output node ofinverter 202C, and the QB-output of register circuit 204B and the outputnode of inverter 202C are gated to NOR gate 205B. A CLR input ofregister circuit 204B is controlled by output node signal N11 ofinverter 202B. In response to a rising edge in the signal at the CLRinput of each register circuit 204A-204B, the signal at the QB-output ofthat register circuit is driven to a logic high state. The D-input ofregister circuit 204B is tied to power supply V_(DD). The D input ofregister circuit 206B is tied to power supply V_(SS), and a clock inputof register circuit 206B is coupled to the output of NOR gate 205B. TheQ-output of register circuit 206B is coupled to another input of errorcorrection circuitry 105. The inverting PRE input of register circuit206B is coupled to the output 208 of error correction circuitry 105. ThePRE input of each register circuit 206A-206B is active low, meaning thatthe Q output of each register circuit is preset to a logic high state inresponse to a falling edge in the signal at its PRE input.

In some embodiments, error correction circuitry 105 may be referred toas a delay circuit. For example, as shown in FIG. 2, error correctioncircuitry 105 may include AND gate 212 having inputs coupled to theQ-outputs of registers 206A-206B and an output that couples to an inputof an inverter chain having an even number of inverters that includesinverter circuit 214. Inverter circuit 214 may be part of a group ofinverters that are serially connected. Error correction circuitry 105receives error detection circuit outputs 230A and 230B as inputs andgenerates error correction output 208 at the output of the last invertercircuit in the chain of inverters. Error correction output 208 maycontrol the inverting PRE inputs of register circuit 206A of errordetection circuit 203A and register circuit 206B of error detectioncircuit 203B to produce updated error detection circuit outputs 230A and230B.

In response to one of error detection circuits 203A or 203B detecting anerror signal in ring oscillator circuit 101 as described above, theerror detection circuit 203A or 203B that detects the error signaladjusts the logic state of its respective error detection circuit output230A or 230B. In response to the logic state of either one of errordetection circuit outputs 230A or 230B being adjusted by the respectiveerror detection circuit 203A or 203B, error correction circuitry 105triggers an error correction operation. During the error correctionoperation, error correction circuitry 105 adjusts the logic state oferror correction output 208. In response to the adjustment in the logicstate of error correction output 208, the error detection circuit 203Aor 203B that detected the error signal adjusts the logic state of itsoutput again to generate a respective updated error detection circuitoutput 230A or 230B. Subsequently, the resulting updated error detectioncircuit outputs 230A and 230B are logically combined by NAND gate 220 toreset the IN signal that has passed through the loop. In one embodiment,the IN signal, after being reset, may represent an initial logic stateof the IN signal during an initialization phase of integrated circuit100. This assures an error-free oscillation of the IN signal in ringoscillator circuit 101. For example, the initial logic state of the INsignal is at a logic value of “1”.

FIG. 3 is a timing diagram 300 illustrating the behavior of some of thesignals in the circuitry of FIGS. 1 and 2 in accordance with oneembodiment of the present invention. It should be appreciated that theembodiments of FIGS. 1 and 2 may be used as examples to describe thetiming diagram 300. As shown, timing diagram 300 illustrates the outputnode signal of inverter 202A (e.g., output node signal N1), output nodesignal of inverter 202C (e.g., output node signal N21), output signal ofregister circuit 204A (e.g., signal QB), output signal of NOR gate 205A(e.g., signal 207A), error detection circuit output 230A, and errorcorrection output 208.

When integrated circuit 100 is powered and operational, an input triggersignal (e.g., IN signal) that is injected into integrated circuit 100may propagate or oscillate through ring oscillator circuit 101 at apredetermined frequency, as shown by output node signals N1 and N21between times T₁ and T₂. When an error event (i.e., harmonic-inducederror) is triggered within ring oscillator circuit 101, error detectioncircuit 203A may detect the location of the error event in ringoscillator circuit 101 by tapping output node signal N1 at the outputnode of inverter 202A and the output node signal N21 at the output nodeof inverter 202C. If an error event occurs between the output nodes ofinverters 202A and 202C, a rising edge (i.e., the change from a logiclow state to a logic high state) occurs in signal N1 followed by afalling edge in signal N1 without an intervening rising edge occurringin signal N21. This scenario is illustrated in FIG. 3 which shows arising edge in output node signal N1 at time T₂ and falling edge insignal N1 at time T₃ without an intervening rising edge in signal N21occurring between these two edges in signal N1. In response to therising edge in signal N1 at time T₂, signal N1 triggers register circuit204A to latch power supply V_(DD) and to generate a falling edge at itsinverting QB-output at time T₂. The QB-output of register circuit 204Aand output node signal N1 are gated to NOR gate 205A.

In response to the QB-output of register circuit 204A and output nodesignal N1, NOR gate 205A generates signal 207A, which is provided toregister circuit 206A as a clock input. In response to the falling edgein signal N1 at time T₃ and the logic low level state at the QB-outputof register circuit 204A, NOR gate 205A generates a rising edge insignal 207A. Just after time T₃, register circuit 206A generates afalling edge in error detection circuit output 230A by latching powersupply V_(SS) to its Q output in response to the rising edge in signal207A to indicate that the error event has occurred in ring oscillatorcircuit 101. For example, just after time T₃, error detection circuitoutput 230A is at the logic low level (e.g., “0”). In this situation,error detection circuit output 230A disables the operation of ringoscillator (RO) circuit 101 while output 230A is at the logic low levelstate.

Accordingly, error correction circuitry 105 receives error detectioncircuit output 230A and generates error correction output 208. As shownin timing diagram 300, a falling edge occurs in the error correctioncircuit output 208 to a logic low level (e.g., “0”) at time T₄ inresponse to the falling edge in signal 230A at time T₃. In oneembodiment, error correction output 208 may control the inverting PREinputs of register circuit 206A of error detection circuit 203A andregister circuit 206B of error detection circuit 203B to produce updatederror detection circuit outputs 230A and 230B. For example, at time T₅,register circuit 206A generates a rising edge in the updated errordetection circuit output 230A to a logic high level (e.g., “1”) inresponse to the falling edge in output 208 at time T₄ presetting circuit206A. Error correction circuitry 105 generates a rising edge in output208 at time T₆ in response to the rising edge in output 230A a period oftime after the rising edge in output 230A occurs.

In response to the rising edge in output 230A at time T₅, NAND gate 220re-enables ring oscillator 101 to allow oscillations again. In thisscenario, output 230A is logically combined by NAND gate 220 to resetthe input trigger signal that has passed through ring oscillator circuit101 for a period of time (e.g., at the time interval between T₄ and T₆).In one embodiment, the input trigger signal, after being reset, mayrepresent an initial state of the input trigger signal during aninitialization phase of integrated circuit 100. This assures anerror-free oscillation of the input trigger signal in ring oscillatorcircuit 101, as shown by output node signals N1 and N21 at the timeinterval between T₅ and T₇.

Similar to the operation described above with respect to error detectioncircuit 203A, error detection circuit 203B may detect an error event inring oscillator circuit 101. If an error event occurs between the outputnodes of inverters 202B and 202C, a rising edge occurs in signal N21followed by a falling edge in signal N21 without an intervening risingedge occurring in signal N11. Circuit 203B generates a falling edge inerror detection circuit output 230B in response to detecting this errorevent. The operation of ring oscillator circuit 101 is disabled whileoutput 230B is at the logic low state. In response to the falling edgein output 230B, error correction circuitry 105 generates a falling edgein output 208 after the delay of circuitry 105. In response to thefalling edge in output 208, a rising edge is generated in output 230B,which causes a subsequent rising edge in output 208. The rising edge inoutput 230B re-enables ring oscillator circuit 101.

FIG. 4 is a flow chart of illustrative steps for detecting andcorrecting errors on an integrated circuit in accordance with oneembodiment of the present invention. It should be appreciated that theembodiments of FIGS. 1 and 2 may be used as examples implemented by thesteps described below.

At step 401, first, second, and third output node signals are tappedfrom a signal that oscillates in a ring oscillator circuit usingmonitoring circuitry. For example, referring to FIGS. 1 and 2, whenintegrated circuit 100 is powered and operational, an input triggersignal (e.g., IN signal) that is injected into ring oscillator circuit101 may oscillate through the ring at a predetermined frequency. Inorder to detect error events that occur in the input trigger signal,ring oscillator circuit 101 may be tapped at different circuit stages(e.g., inverters 202A, 202B, and 202C) using monitoring circuitry 103.For example, error detection circuit 203A of monitoring circuitry 103may detect an error signal if an error event occurs between the outputnode of inverter 202A and the output node of inverter 202C, as describedabove. Similarly, error detection circuit 203B of monitoring circuitry103 may detect another error signal that indicates that an error eventhas occurred between the output node of inverter 202B and the outputnode of inverter 202C, as described above.

At step 402, first and second error detection circuit outputs aregenerated based on the first, second, and third output node signalsusing first and second error detection circuits. For example, as shownin FIG. 2, error detection circuit 203A may output error detectioncircuit output 230A in response to the error signal that is detectedbased on output node signals N1 and N21, and error detection circuit203B may output error detection circuit output 230B in response to theanother error signal that is detected based on output node signals N11and N21.

At step 403, an error correction output is generated based on the firstand second error detection circuit outputs using an error correctioncircuit. Referring to FIG. 2, error correction circuitry 105 receiveserror detection circuit outputs 230A and 230B from the respective errordetection circuits 203A and 203B as inputs and generates errorcorrection output 208. In one embodiment, when any of error detectioncircuit outputs 230A and 230B is asserted (e.g., its logic statechanges), this event may trigger an error correction operation in errorcorrection circuitry 105. In this scenario, error correction circuitry105 may generate error correction output 208. For example, errorcorrection circuitry 105 may adjust the logic state of output 208 inresponse to a change in the logic state of output 230A or 230B. In oneembodiment, error correction output 208 may control the inverting PREinputs of register circuit 206A of error detection circuit 203A andregister circuit 206B of error detection circuit 203B to produce updatederror detection circuit outputs 230A and 230B at step 404.

At step 405, the signal that has passed through the ring oscillator isreset based on the first and second updated error detection circuitoutputs using a logic circuit. For example, as shown in FIG. 2, theresulting updated error detection circuit outputs 230A and 230B of errordetection circuits 203A and 203B are logically combined by a logiccircuit (e.g., NAND gate 220) to reset the IN signal that has passedthrough the loop. The IN signal, after being reset, may represent aninitial state of the IN signal during a power up operation of aninitialization phase of integrated circuit 100. This assures anerror-free oscillation of the IN signal in ring oscillator circuit 101.

The present exemplary embodiments may be practiced without some or allof these specific details described with reference to the respectiveembodiments. In other instances, well-known operations have not beendescribed in detail in order not to obscure unnecessarily the presentembodiments.

The methods and apparatuses described herein may be incorporated intoany suitable circuit. For example, the methods and apparatuses may beincorporated into numerous types of devices such as microprocessors orother integrated circuits. Exemplary integrated circuits includeprogrammable array logic (PAL), programmable logic arrays (PLAs), fieldprogrammable logic arrays (FPGAs), electrically programmable logicdevices (EPLDs), electrically erasable programmable logic devices(EEPLDs), logic cell arrays (LCAs), field programmable gate arrays(FPGAs), application specific standard products (ASSPs), applicationspecific integrated circuits (ASICs), and microprocessors, just to namea few.

Although the method operations were described in a specific order, itshould be understood that other operations may be performed in betweendescribed operations, described operations may be adjusted so that theyoccur at slightly different times or described operations may bedistributed in a system which allows the occurrence of the processingoperations at various intervals associated with the processing, as longas the processing of the overlay operations are performed in a desiredway.

What is claimed is:
 1. A circuit comprising: a ring oscillator circuitcomprising a plurality of inverters in a loop, wherein the plurality ofinverters comprises first, second, and third output nodes; monitoringcircuitry that monitors for an error event in a signal that has passedthrough the inverters from any one of the first, second, or third outputnodes, wherein the monitoring circuitry selectively resets the signal inresponse to the error event, and wherein the error event comprises aharmonic error event, and wherein the monitoring circuitry comprises: afirst error detection circuit that receives first output signals fromthe first and third output nodes and generates a first error detectioncircuit output, and a second error detection circuit that receivessecond output signals from the second and third output nodes andgenerates a second error detection circuit output; and an errorcorrection circuit that receives the first and second error detectioncircuit outputs and produces an error correction output based on thefirst and second error detection circuit outputs, wherein the firsterror detection circuit receives the error correction output andproduces a first updated error detection circuit output in response tothe error correction output, and wherein the second error detectioncircuit receives the error correction output and produces a secondupdated error detection circuit output in response to the errorcorrection output.
 2. A circuit comprising: a ring oscillator circuitcomprising a plurality of inverters in a loop, wherein the plurality ofinverters comprises first, second, and third output nodes; andmonitoring circuitry that monitors for an error event in a signal thathas passed through the inverters from any one of the first, second, orthird output nodes, wherein the monitoring circuitry selectively resetsthe signal in response to the error event, wherein the error eventcomprises a harmonic error event, and wherein the monitoring circuitrycomprises: a first error detection circuit that receives first outputsignals from the first and third output nodes and generates a firsterror detection circuit output, wherein the first error detectioncircuit comprises first and second register circuits and a first logicgate coupled between the first and second register circuits; and asecond error detection circuit that receives second output signals fromthe second and third output nodes and generates a second error detectioncircuit output, wherein the second error detection circuit comprisesthird and fourth register circuits and a second logic gate coupledbetween the third and fourth register circuits.
 3. The circuit definedin claim 2 further comprising: an error correction circuit that receivesthe first and second error detection circuit outputs and produces anerror correction output based on the first and second error detectioncircuit outputs.
 4. The circuit defined in claim 3, wherein the ringoscillator circuit further comprises: control logic that is interposedin the loop and that has a first input that is connected to an inverteroutput in the loop, and wherein the control logic has an outputconnected to an inverter input in the loop.
 5. The circuit defined inclaim 4, wherein the first error detection circuit produces a firstupdated error detection circuit output in response to the errorcorrection output, and wherein the second error detection circuitproduces a second updated error detection circuit output in response tothe error correction output.
 6. The circuit defined in claim 5, whereinthe control logic has second inputs that receive the first and secondupdated error detection circuit outputs, and resets the signal that haspassed through the inverters based on the first and second updated errordetection circuit outputs.
 7. An integrated circuit comprising: a ringoscillator circuit comprising a plurality of inverters in a loop,wherein the plurality of inverters comprises first, second, and thirdoutput nodes; monitoring circuitry that monitors for an error event in asignal that has passed through the inverters from any one of the first,second, or third output nodes, wherein the monitoring circuitryselectively resets the signal in response to the error event, whereinthe error event comprises a harmonic error event, and wherein themonitoring circuitry comprises: a first error detection circuit thatreceives first output signals from the first and third output nodes andgenerates a first error detection circuit output, and a second errordetection circuit that receives second output signals from the secondand third output nodes and generates a second error detection circuitoutput; and an error correction circuit that receives the first andsecond error detection circuit outputs and produces an error correctionoutput based on the first and second error detection circuit outputs,wherein the first error detection circuit produces a first updated errordetection circuit output in response to the error correction output,wherein the second error detection circuit produces a second updatederror detection circuit output in response to the error correctionoutput, wherein the ring oscillator circuit further comprises a logiccircuit that is interposed in the loop, and wherein the logic circuitlogically combines the first and second updated error detection circuitoutputs to reset the signal that has passed through the inverters. 8.Circuitry comprising: a ring oscillator circuit having first, second,and third output nodes, wherein each of the first, second, and thirdoutput nodes taps a signal that has passed through the ring oscillatorcircuit, wherein the ring oscillator circuit comprises a plurality ofinverters in a loop, and wherein the first output node is coupled to afirst inverter output in the loop, the second output node is coupled toa second inverter output in the loop, and the third output node iscoupled to a third inverter output in the loop; error detectioncircuitry that detects an error event in the signal through the first,second, and third output nodes, wherein the error event comprises aharmonic error event; and an error correction circuit, wherein the errordetection circuitry comprises: a first error detection circuit thatgenerates a first error detection circuit output indicating whether theerror event has occurred between the first and third output nodes, and asecond error detection circuit that generates a second error detectioncircuit output indicating whether the error event has occurred betweenthe second and third output nodes, wherein the error correction circuitreceives the first and second error detection circuit outputs andproduces an error correction output that indicates when either of thefirst or second error detection circuit outputs indicates the errorevent, wherein the first error detection circuit produces a firstupdated error detection circuit output in response to the errorcorrection output, wherein the second error detection circuit produces asecond updated error detection circuit output in response to the errorcorrection output, wherein the ring oscillator circuit is disabled inresponse to either of the first or second error detection circuitoutputs indicating the error event, and wherein one of the first orsecond updated error detection circuit outputs enables the ringoscillator circuit to allow oscillations again.
 9. The circuitry definedin claim 8, wherein the first error detection circuit comprises firstand second register circuits and a logic gate coupled between the firstand second register circuits.
 10. The circuitry defined in claim 8,wherein the ring oscillator circuit further comprises a logic circuitthat is interposed in the loop, wherein the logic circuit logicallycombines the first and second updated error detection circuit outputs toreset the signal that has passed through the loop.
 11. The circuitrydefined in claim 10, wherein the logic circuit comprises a NAND gate.12. A method for detecting and correcting error events in a ringoscillator, the method comprising: with error detection circuitry,monitoring error effects associated with a signal that propagatesthrough the ring oscillator by detecting at least one error conditionfrom first, second, or third output nodes of the ring oscillator; withan error correction circuit coupled to the error detection circuitry,correcting the error effects associated with the signal when the atleast one error condition is detected in either the first output node,the second output node, or the third output node of the ring oscillator,wherein the at least one error condition comprises a harmonic errorevent, and wherein monitoring error effects associated with the signalcomprises: with a first error detection circuit in the error detectioncircuitry, generating a first error detection circuit output when the atleast one error condition is detected between the first and third outputnodes of the ring oscillator, and with a second error detection circuitin the error detection circuitry, generating a second error detectioncircuit output when the at least one error condition is detected betweenthe second and third output nodes of the ring oscillator; with the errorcorrection circuit, receiving the first and second error detectioncircuit outputs to generate an error correction output; with the firsterror detection circuit, generating a first updated error detectioncircuit output based on the error correction output; and with the seconderror detection circuit, generating a second updated error detectioncircuit output based on the error correction output.
 13. The methoddefined in claim 12, wherein the ring oscillator comprises a pluralityof inverters in a loop, and wherein correcting the error effectsassociated with the signal comprises: with a logic circuit coupled inthe loop, receiving the first and second updated error detection circuitoutputs, and resetting the signal that has passed through the pluralityof inverters based on the first and second updated error detectioncircuit outputs.